Process for preparing crystalline ammonium dihydrogen phosphate

ABSTRACT

A process for preparing crystalline ammonium dihydrogen phosphate, comprises the steps of (a) adding a sequestering agent into ammonium hydrogen phosphate aqueous solution, the sequestering agent reacting with metallic ions contained in the aqueous solution to form water soluble compounds, (b) heat-treating the aqueous solution at a temperature of 70° C. or more after addition of the sequestering agent, and (c) crystallizing ammonium dihydrogen phosphate out of the aqueous solution after the heat-treating, so that crystalline ammonium dihydrogen phosphate can be obtained in high whiteness and in the form of columnar crystals.

BACKGROUND OF THE INVENTION

This invention relates to a process for preparing crystalline ammoniumdihydrogen phosphate (NH₄ H₂ PO₄) which is high in whiteness and in thecolumnar form, and more particularly to the same process for preparingammonium dihydrogen phosphate from wet process phosphoric acid.

In accordance with a conventional process for preparing ammoniumdihydrogen phosphate from wet process phosphoric acid, a formedcrystalline ammonium dihydrogen phosphate is in the form of needlecrystal which is colored by the effect of various impurities. Theseimpurities include organic compounds and inorganic metallic ions.

The organic compounds may be removed to a considerable extent by means,for example, of oxidation or reduction treatment, or active carbontreatment. However, the metallic ions are considerably difficult to beremoved even by such a treatment, because they are relatively liable toenter the crystal of ammonium dihydrogen phosphate and tend to beconcentrated by some treating manners. Additionally, the crystal form ofammonium dihydrogen phosphate becomes needle-like by the effect of suchimpurities.

In this regard, it has also been proposed to repeat therecrystallization of ammonium dihydrogen phosphate crystal, by which theimpurities in the crystal of the ammonium dihydrogen phosphate can beremoved to some extent. However, such a proposition encounters thefollowing disadvantages from point of view of industrial manufacturing:not only process is complicated, but also the yield of ammoniumdihydrogen phosphate is liable to decrease, contributing to economicaldisadvantage. Additionally, even by this proposition, the crystal formof ammonium dihydrogen phosphate can not improved to a desired extent.

Ammonium dihydrogen phosphate is usually crystallized in the needle-likeform in case where the phosphate is prepared by the reaction of wetprocess phosphoric acid and ammonia. Such needle-like crystal isdisadvantageous because it is not effective for filtration, and bulkdensity is considerably low increasing storing and wrapping costs.Furthermore, the needle-like crystal is relatively liable to be blockedin a container.

Now, such crystalline ammonium dihydrogen phosphate is used as thestarting materials for various phosphates, baking powder, boilercompounds, decoloring agent for wool, powdered fire extinguishing agentsand food additives. Of these, the use as the fire extinguishing agent isrecently increasing, the powder of which agent is required to be uniformand in the form of fine particles. It is to be noted that as thepowdered fire extinguishing agent, columnar or granulated crystal isdesirable because the needle-like crystal is not suitable to be injectedfrom a fire extinguisher.

SUMMARY OF THE INVENTION

In view of the above, the present invention contemplates obtainingcrystalline ammonium dihydrogen phosphate in the desired color and form,by preventing inorganic ions contributing to coloring crystal fromentering the crystal, in which the inorganic ions remain in motherliquor forming chelate compounds.

It is the principal object of the present invention to provide animproved process for preparing crystalline ammonium dihydrogen phosphatewhich is in the color and in the crystal form suitable for recentvarious uses and handling, without inviting economical disadvantages.

It is another object of the present invention to provide an improvedprocess for preparing crystalline ammonium dihydrogen phosphate which ishigh in whiteness and generally in columnar crystal form.

It is a further object of the present invention to provide an improvedprocess for preparing crystalline ammonium dihydrogen phosphate in theform of columnar crystal, by which the impurities contained in ammoniumdihydrogen crystal are effectively removed without repetition ofrecrystallization.

It is a still further object of the present invention to provide animproved process for preparing crystalline ammonium dihydrogenphosphate, in which metallic ions contributing coloring crystals areeffectively removed by preventing the formation of their metallic salts,which is accomplished by forming the chelate compounds between themetallic ions and a sequestering agent.

These and other objects, features and advantages of the process inaccordance with the present invention will be more apparent from thefollowing description.

PREFERRED EMBODIMENTS OF THE INVENTION

In accordance with the process of the present invention, at least a kindof sequestering agent is added into ammonium dihydrogen phosphateaqueous solution, in which the sequestering agent reacts with metallicions contained in the aqueous solution to form water soluble compounds.The ammonium dihydrogen phosphate aqueous solution is purified by activecarbon treatment prior to addition of the sequestering agent in order toremove almost all organic compounds contained in the aqueous solution,if necessary. Then, the aqueous solution to which the sequestering agentis added is heat-treated at a temperature of 70° C. or more. Thereafter,the crystallization of ammonium dihydrogen phosphate is carried out inthe aqueous solution.

As the above-mentioned sequestering agent for forming water-solublecompound with metallic ions contained in the ammonium dihydrogenphosphate aqueous solution, at least one of a sequestering agent ofwater-soluble salt of polyaminocarboxylic acid and a sequestering agentof a condensed phosphate is used. The sequestering agent of thewater-soluble salt of polyaminocarboxylic acid is ethylenediaminetetraacetate (E. D. T. A), nitrilotriacetate (N. T. A) ordiethylenetrinitrilo pentaacetate (D. T. P. A). The sequestering agentof condensed phosphate is sodium pyrophosphate (Na₄ P₂ O₇), sodiumtripolyphosphate (Na₅ P₃ O₁₀), sodium hexametaphosphate (NaPO₃)₆, or theammonium salt of each of them. As appreciated from the above, it isessential to use the sequestering agent in the process according to thepresent invention.

In order to accomplish the process of the present invention, at firstwet process phosphoric acid solution is purified by a known method suchas using active carbon to remove the impurities such as organiccompounds etc. Subsequently, the purified wet process phosphoric acidsolution is contacted with ammonia so as to react with each other untilN/P mol ratio reaches to a value of 0.8/1 to 1.5/1, preferably 1.05/1 to1.2/1 to obtain about 50 wt. percent ammonium dihydrogen phosphateaqueous solution which is then separated by filtration to remove theimpurities such as Fe, Al etc. Thereafter, the sequestering agent isadded into the filtrate of ammonium dihydrogen phosphate, and thefiltrate is then heat-treated at 70° C. or more and for more and for 5minutes or more, preferably 10 to 60 minutes, by which inorganiccoloring ions react with the sequestering agent to form water-solublechelate compounds. Lastly, ammonium dihydrogen phosphate is crystallizedout of the aqueous solution by a known crystallization method to obtaincolumnar crystal of ammonium dihydrogen phosphate which is excellent inwhiteness, since the above-mentioned chelate compounds remain in amother liquor.

With respect to the reaction temperature and time for forming thechelate compounds, if the temperature lower than 70° C. and the time isless than 5 minutes, the chelation is insufficient and therefore it isdifficult to attain sufficient decoloring effect and obtain columnarcrystal.

As appreciated from the above, it is essential to use the sequesteringagent in the process according to the present invention. It is to benoted that, of the above-mentioned various sequestering agents, E. D. T.A is particularly high in ability for causing the chelation andconsequently even a slight amount of E. D. T. A is effective for suchchelation. Additionally, the amount of E. D. T. A required for thechelation is sufficient to be less, as the N/P mol ratio becomes higher.

With respect to the amount of the sequestering agent to be added toammonium dihydrogen phosphate, it is variable depending upon startingmaterial phosphoric acids, and it is preferable that about 0.2 wt.percent or more sequestering agent is added to about 50 wt. percentammonium dihydrogen phosphate aqueous solution (about 0.1 wt. percentsequestering agent relative to the crystals of ammonium dihydrogenphosphate). More preferably, 0.2 to 2 wt. percent sequestering agent isadded to about 50 wt. percent ammonium dihydrogen phosphate aqueoussolution (0.1 to 1 wt. percent sequestering agent relative to thecrystals of ammonium dihydrogen phosphate). In this regard, if the addedamount of the sequestering agent is less than 0.2 wt. percent, chelationeffect is inferior, whereas even if the added amount is more than 2percent, much improved chelation effect cannot be expected.

The present invention will be illustrated in detail by the followingexamples.

EXAMPLE 1

Soda ash was added to wet process phosphoric acid having a P₂ O₅concentration of about 27% which phosphoric acid was obtained fromFlorida phosphate rock, to precipitate fluorine compounds as sodiumsilicofluoride. The precipitate of the sodium silicofluoride was thenseparated by filtration using a vacuum filter to obtain filtrate.Subsequently, gaseous ammonia was blown into the filtrate so that the pHvalue was adjusted to 4.6 to form the precipitate of hydroxide. Theprecipitate of hydroxide was then removed by a centrifugal separator toobtain 46 wt. percent ammonium dihydrogen phosphate aqueous solution.0.2 wt. percent active carbon was added into the ammonium dihydrogenphosphate aqueous solution, and subsequently the active carbon wasseparated by filtration after aging to remove organic compounds. E. D.T. A.2Na.2H₂ O was added to the filtration of ammonium dihydrogenphosphate aqueous solution, in which the amount of E. D. T. A was variedrelative to the weight of the ammonium dihydrogen phosphate aqueoussolution in the state before the treatment with active carbon. Then, theammonium dihydrogen phosphate aqueous solution was kept at 90° C. ormore and for 20 minutes, and thereafter the aqueous solution was cooleduntil 35° C. at a rate of 1° C./minute in a container equipped with anagitator to crystallize ammonium dihydrogen phosphate. The thus formedcrystal was separated by a small-sized centrifugal separator and thenwashed with a small amount of ammonium dihydrogen phosphate saturatedsolution. As a result, columnar crystals of ammonium dihydrogenphosphate which are high in whiteness are obtained by theabove-mentioned addition of 0.2 wt. percent or more E. D. T. A.2Na.2H₂ Oas shown in Table 1 mentioned below,

                  TABLE 1                                                         ______________________________________                                        E.D.T.A. · 2Na · 2H.sub.2 O                                                          crystal                                              added amount (%)                                                                           color       form       whiteness                                 ______________________________________                                        0            greenish blue                                                                             needle-like                                                                              91.5                                                   light greenish                                                   0.10         blue        needle-like                                                                              94.2                                                   white(slightly                                                   0.20         colored)    columnar   97.2                                      0.50         white       columnar   98.6                                      0.75         white       columnar   99.2                                      ______________________________________                                         Note:                                                                         (1) the needlelike form is 5/1 or higher in the ratio of the major            axis/the minor axis, whereas the columnar form is lower than 5/1 in the       same ratio.                                                                   (2) the whiteness is relative values to a standard value (100.0) of a         reagent ammonium dihydrogen phosphate. The relative values were determine     according to JIS (Japanese Industrial Standard) Z 8730.                  

EXAMPLE 2

0.75 wt. percent E. D. T. A.2Na.2H₂ O was added to 900 g of 46 wt.percent ammonium dihydrogen phosphate aqueous solution which was thesame as used in EXAMPLE 1 and obtained after the treatment with activecarbon. Then, the aqueous solution was heat-treated at 90° C. or moreand for 30 minutes, and subsequently concentrated at 50° C. under vacuumuntil the amount of evaporated water reached to 250 g, immediately afterwhich the crystal of ammonium dihydrogen phosphate was separated by asmall-sized centrifugal separator. The separated crystal was washed witha small amount of ammonium dihydrogen phosphate saturated solution toobtain 203 g of crystalline ammonium dihydrogen phosphate. The thusobtained crystal was columnar and white exhibiting the whiteness of99.0. For comparison, a comparative ammonium dihydrogen phosphate wasprepared by the same process as mentioned above with the exception thatno metallic ion blocking agent was added. The resultant comparativeammonium dihydrogen phosphate was needle-like and greenish blue crystalwhich exhibits the whiteness of 89.3.

EXAMPLE 3

A variety of rates of N. T. A.Na₃.H₂ O were added to 46 wt. percentammonium dihydrogen phosphate which was the same as used in EXAMPLE 1and obtained after the treatment with active carbon. Then, the aqueoussolution was kept at 80° C. and for 30 minutes and thereafter treatedsimilarly to in EXAMPLE 1 to obtain crystalline ammonium dihydrogenphosphate. The whiteness and form of the resultant crystals of ammoniumdihydrogen phosphate are shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        N.T.A. · Na.sub.3 · H.sub.2 O                                                        crystal                                              added amount (%)                                                                           color       form       whiteness                                 ______________________________________                                        0            greenish blue                                                                             needle-like                                                                              91.5                                      0.1          greenish blue                                                                             needle-like                                                                              92.9                                      0.2          light blue  needle-like                                                                              93.6                                                   white(slightly                                                   0.5          colored)    columnar   96.4                                      1.0          light green columnar   94.0                                      2.0          light blue  needle-like                                                                              92.7                                      ______________________________________                                    

EXAMPLE 4

A variety of rates of sodium tripolyphosphate was added to 46 wt.percent ammonium dihydrogen phosphate aqueous solution which was thesame as used in EXAMPLE 1 and obtained after treatment with activecarbon. Then, the aqueous solution was kept at about 80° C. and for 40minutes and thereafter treated similarly to in EXAMPLE 1 to obtaincrystalline ammonium dihydrogen phosphate. The whiteness and the form ofthe resultant crystals of ammonium dihydrogen phosphate are shown inTable 3 below.

                  TABLE 3                                                         ______________________________________                                        Na.sub.5 P.sub.3 O.sub.10                                                                             crystal                                               added amount(%)                                                                           color       form       whiteness                                  ______________________________________                                        0           greenish blue                                                                             needle-like                                                                              91.5                                       0.2         greenish blue                                                                             needle-like                                                                              92.3                                       1.0         greenish blue                                                                             needle-like                                                                              92.8                                       2.0         greenish blue                                                                             needle-like                                                                              93.5                                       5.0         bright green                                                                              needle-like                                                                              94.6                                       ______________________________________                                    

EXAMPLE 5

A variety of rates of E. D. T. A.2Na.2H₂ O and sodium pyrophosphate wasadded to 46 wt. percent ammonium dihydrogen phosphate which was the sameused in EXAMPLE 1 and obtained after the treatment with active carbon.Then, the aqueous solution was kept at about 95° C. and for about 30minutes and thereafter treated similarly to in EXAMPLE 1 to obtaincrystalline ammonium dihydrogen phosphate. The whiteness and the form ofthe resultant crystal of ammonium dihydrogen phosphate are shown inTable 4 below.

                  TABLE 4                                                         ______________________________________                                                                  crystal                                             added amount (%) color    form      whiteness                                 ______________________________________                                        E.D.T.A. · 2Na · 2H.sub.2 O                                               0.2                                                                                  white    columnar                                                                              96.8                                    sodium pyrophosphate                                                                        0.2                                                             E.D.T.A. · 2Na · 2H.sub.2 O                                               0.5                                                                                  white    columnar                                                                              99.0                                    sodium pyrophosphate                                                                        0.2                                                             ______________________________________                                    

What is claimed is:
 1. A process for preparing crystalline ammoniumdihydrogen phosphate from an aqueous solution of ammonium dihydrogenphosphate obtained from wet process phosphoric acid, comprising thesteps of:(a) adding ethylenediamine tetraacetate into the wet processammonium dihydrogen phosphate aqueous solution, the amount of saidethylenediamine tetraacetate added being within a range of from 0.1 to 1wt. percent relative to crystals of ammonium dihydrogen phosphate; (b)heat-treating said aqueous solution at a temperature of 70° C. or moreand for a time within a range of from 10 to 60 minutes, after additionof said ethylenediamine tetraacetate; and (c) crystallizing ammoniumdihydrogen phosphate out of said aqueous solution after saidheat-treating.
 2. A process as claimed in claim 1, including removingimpurities contained in the ammonium dihydrogen phosphate aqueoussolution to purify it, before carrying out step (a).
 3. A process asclaimed in claim 2, wherein the step of removing impurities includestreating the ammonium dihydrogen phosphate aqueous solution with activecarbon to remove organic compounds contained in the aqueous solution.